Mechanical alloying is a powder metallurgy process consisting of repeatedly welding, fracturing and rewelding powder particles through high energy mechanical milling. Mechanochemical processing is the application of mechanical alloying techniques to chemical refining through sold state reactions. The energy of impact of the milling media, the balls in a ball mill for example, on the reactants is effectively substituted for high temperature so that solid state reactions can be carried out at room temperature. Although a number of elemental and alloy powders have been easily produced using mechanochemical processing techniques, the production of titanium has been problematic due to long milling times and the contamination associated with the long milling times.
Titanium and its alloys are attractive materials for use in aerospace and terrestrial systems. There are impediments, however, to wide spread use of titanium based materials in, for example, the cost conscious automobile industry. The titanium based materials that are commercially available now and conventional techniques for fabricating components that use these materials are very expensive. Titanium powder metallurgy, however, offers a cost effective alternative for the manufacture of titanium components if low cost titanium powder and titanium alloy powders were available. The use of titanium and its alloys will increase significantly if they can be inexpensively produced in powder form.
Currently, titanium powder and titanium alloy powders are produced by reducing titanium chloride through the Kroll or Hunter processes and hydrogenating, crushing and dehydrogenating ingot material (the HDH process). The cost of production by these processes is much higher than is desireable for most commercial uses of titanium powders. In the case of titanium alloy powders, especially multi-component alloys and intermetallics, the cost of HDH production escalates because the alloys must generally be melted and homogenized prior to HDH processing.
Presently, the production of titanium by reducing titanium chloride is a multi-step process. First titanium oxide is converted to titanium chloride in the presence of carbon at high temperature, as shown in Eq. 1. EQU TiO.sub.2 +2Cl.sub.2 (in the presence of carbon at high temperature).fwdarw.TiCl.sub.4 (1)
Then, the titanium chloride is reduced by magnesium at a temperature above 800.degree. C. Magnesium chloride MgCl.sub.2 is a by-product of the reaction in this process, which is shown in Eq. 2. EQU TiCl.sub.4 +2Mg.fwdarw.Ti+2MgCl.sub.2 (2)
The magnesium chloride MgCl.sub.2 is removed by leaching or vacuum distilling to low levels to get sponge titanium. The powder or "sponge fines" is the small size faction of the sponge. Leaching is carried out by dissolving the unreacted magnesium using a mixture of hydrochloric HCl and 10% nitric HNO.sub.3 acids followed by several washings with water. The cost of producing titanium powder this way is high because of the large consumption of energy, problems associated with the high temperatures and the difficulties in removing magnesium chloride MgCl.sub.2.
A number of attempts have been made in the past to reduce the cost of producing titanium sponge. These include continuous injection of titanium chloride into a molten alloy system consisting of titanium, zinc and magnesium, vapor phase reduction and aerosol reduction. Although cost reductions as high as 40% have been estimated for some of these techniques, a common feature of all of these processes is the use of high temperatures to reduce titanium chloride or titanium oxide.
Apart from cost, production of titanium base alloys present another important problem with regard to their brittleness. The use of high temperature titanium aluminides prepared by conventional techniques is limited by low ductility. Recent work on aluminides has shown that their ductility can be increased considerably by producing the material in nanocrystalline form.